The present invention relates to a planar sensor element having a gas-containing layer situated between impermeable covering layers.
Planar sensor elements of that kind are known and widespread as component parts of lambda sensors. Conventionally, they have impermeable covering layers made of sintered ZrO2 between which an electric heating device is embedded between porous insulating layers made of aluminum oxide. The use of such insulating layers is necessary because the ZrO2 used for the covering layers becomes electrically conductive at the operating temperatures which are usual for these sensor elements, which can result in leakage currents and in a falsification of the obtained measured values or even in the destruction of the sensor element and, consequently, in the failure of the lambda sensor. In contrast, aluminum has a conductivity which is smaller by several orders of magnitude.
In a lambda sensor, the air content of these porous layers can be used as pump volume for a pumped reference or as reference gas space. Also known are designs of planar sensor elements in which the reference gas space forms a hollow space within a layer. Such a layer including a hollow space which can contain the reference gas is understood here as a gas-containing layer, as well.
Usually, the gas-containing layer is brought out toward reference gas at an end face of the planar sensor element, thus forming a narrow strip at the end face via which an air exchange with the ambient environment or with the reference gas can take place.
If a sensor element is not supported absolutely gas and moisture-tight at this end face, there is a risk for moisture or exhaust components to penetrate into the gas-containing layer, resulting in the falsification of the measured values. In particular the ingress of moisture is promoted by the narrow strip shape of the passage of the gas-containing layer.
The necessity of this passage also creates difficulties in the manufacture of the sensor elements. Usually, a plurality of such sensor elements are made on a shared substrate and then separated. In this context, during the cutting of the substrate along an end face including the passage of the gas-containing layer, the edges of the substrate can easily flake off, or the notch effects arising during cutting result in imperfections at the substrate edge which, during subsequent sintering, prevent the formation of a composite having a satisfactory quality.
An improved mechanical strength of a sensor element of the type mentioned at the outset is achieved according to the present invention in that the gas-containing layer is enclosed all around by a sealing frame in a plane running between the covering layers instead of on only three sides as is the case with the conventional sensor elements, and in that at least one of the covering layers is provided with a through hole for the required exchange of air between the gas-containing layer of the sensor element and its ambient environment.
Since, during the separation of a substrate having a plurality of such sensor elements prior to sintering, only the material of the sealing frame and of the covering layers needs to be cut, i.e., usually ZrO2, the wear of the cutting tools is also reduced in comparison with the case that a porous insulating layer of Al2O3 needs to be cut.
The closed sealing frame also reduces the probability of damaging the laminate structure of the sensor element when scribing and subsequently breaking the sensors along their adjacent edges during separation.
Conventional sensor elements frequently possess contact surfaces at their supported end which can be used for supplying current to a heating element or for tapping an electric signal from a measuring electrode, and which are contacted via through holes of the covering layers to conductive tracks located inside the layer structure of the sensor element. According to the present invention, these through holes are at the same time used to permit the air exchange between the gas-containing layer and the ambient environment. In this connection, the through holes have, of course, a larger cross section than the conductor which is passed through to permit the passage of gas.
To intensify the effectiveness of the overall exchange between the gas-containing layer and the ambient environment, provision can be made for the free cross-sectional area of the through holes to be larger than the free cross-sectional area of the gas-containing layer transverse to the longitudinal axis of the sensor element. This latter cross-sectional area corresponds to the gas exchange cross-sectional area of an open-edge sensor element of conventional design, as will be described in the following.
Preferably, the electric conductor which is passed through the through hole is used for supplying current to a heating element embedded in the reference-gas containing layer.
The dimensions of the through hole are no longer limited by the cross-sectional area of the gas-containing layer which is why the free diameter of the through hole can be larger than the thickness of the gas-containing layer without any problem, on one hand, to promote an efficient air exchange between the gas-containing layer and the ambient environment and, on the other hand, to prevent the through hole from exerting a strong capillary effect on liquid possibly present in its ambient environment and from drawing it into the gas-containing layer. The diameter of such a through hole should be at least 0.5, preferably approximately 1 mm.